Transforming growth factor alpha (TGFalpha) has been circumstantially implicated in the autocrine growth of a number of different rodent and human tumor cells in vitro. However, its distribution and role in the etiology and/or progression of rodent and human breast cancer are relatively unknown. The present studies have demonstrated that biologically active and immunoreactive TGFalpha can be detected in hormone (estrogen)-dependent, DMBA- or NMU-induced rat mammary adenocarcinomas and that these tumors possess a specific 4.8-kb TGFalpha mRNA. These tumors also express elevated levels of c-Ha-ras protein (DMBA tumors) or possess a point-mutated c-Ha-ras gene (NMU tumors). Ovariectomy results in tumor regression and is preceded by a specific decrease in TGFA mRNA and protein. Transformation of nontransformed mouse mammary epithelial cell lines with a point-mutated c-Ha-ras protooncogene results in the loss in mitogenic responsiveness of these cells to exogenous epidermal growth factor (EGF) and a reduction in the number of unoccupied EGF receptors on these cells due in part to an enhanced production and secretion of TGFalpha. Several human breast cancer cell lines also secrete TGFalpha and possess TGFalpha mRNA. In the estrogen-responsive breast cancer cell lines, estrogen can induce a 3- to 5-fold increase in TGFalpha mRNA and protein. Treatment of these cells with a polyclonal anti-TGFalpha antibody or with a monoclonal anti-EGF receptor antibody can inhibit the growth of these cells in vitro. Elevated levels of TGFalpha protein or expression of TGFalpha mRNA can be detected in 50-60% of primary human breast tumors. No evidence was found for any gross amplifications and/or rearrangements of the TGFalpha gene in these tumors. Additionally, overexpression of a human TGFalpha gene in a cloned immortalized population of mouse and human mammary epithelial cells can lead to the malignant transformation of these cells in vitro and in vivo. Collectively, these results suggest that TGFalpha can function as an autocrine growth factor for a subset of rodent and human breast tumors, that estrogens or activated protooncogenes such as ras can enhance the expression of TGFalpha, and that enhanced production of TGFalpha can lead to the transformation of mammary epithelial cells that have a sufficient complement of functional EGF receptors.